1. Field of the Invention
The present invention relates to a liquid crystal display (hereinafter, referred to as "LCD") device, and more particularly to a liquid crystal display capable of avoiding disconnection of a data bus line caused due to a step (i.e. a height difference between piled gate and data bus lines) existing in an intersection of the gate and data bus lines.
2. Description of the Prior Art
FIG. 1 is a plane view showing the construction of a prior art liquid crystal display device.
Referring to FIG. 1, the prior art liquid crystal display device comprises a substrate 10, a plurality of thin film transistors (hereinafter, referred to as "TFTs") 11 arranged in matrix on the substrate 10, a plurality of pixel electrodes 14 arranged in matrix corresponding to each of the TFTs, a data bus line 13 commonly connected with a plurality of the TFTs in column direction and formed between two adjacent TFTs which are spaced from each other in row direction, and a gate bus line 12 commonly connected with a plurality of the TFTs in row direction and formed between two adjacent TFTs which are spaced from each other in column direction.
Each of the TFTs 11 has a gate electrode 11-1 formed on the substrate 10, a source electrode 11-2 extended over one part of the gate 11-1 and connected to the data bus line 13, and a drain electrode 11-3 extended over the other part of the gate 11-1 and connected to a corresponding pixel electrode 14.
A method for fabricating the prior art LCD device having the above-mentioned structure will be described below.
First, a plurality of TFTs is formed in matrix on an upper glass substrate.
To form a gate electrode 11-1 and a gate bus line 12 on a glass substrate, metallization and photolithograpy are sequentially performed. In the metallization, a metal having a good conductivity such as Cr, Al, AlTa, Ta, Ti or the like, is used to form a metal layer on the glass substrate. By the photolithograpy, a patterned electrode layer, i.e. the gate bus line and gate electrode, is formed on the substrate.
Next, on the gate electrode 11-1 a gate insulating layer (not shown in FIG. 1) is deposited by using PECVD(Plasma Enhanced Chemical Vapor Deposition) or APCVD(Atmospheric Pressure Chemical Vapor Deposition) as well-known in this art. The gate insulating layer is composed of oxide or nitride and serves as a medium to form a capacitor in a channel region which is formed on the gate insulating layer.
After formation of the gate insulating layer, an n.sup.+ type amorphous silicon layer is deposited on the gate insulating layer to form the channel region for electron transfer. The amorphous silicon layer can be deposited at temperature less than that a fusing point of the glass substrate. On the gate electrode 11-1, the amorphous silicon layer is patterned to form an active layer (not shown).
Subsequently, a pixel electrode 14 is formed on the gate insulating layer and is spaced from the active layer. Also, an n.sup.+ type amorphous silicon layer (not shown) is formed between the active layer and the pixel electrode 14, thereby allowing an ohmic contact to be formed between the active layer and the source/drain electrode.
Also, at both sides of the active layer source and drain bus lines 11-2, 11-3 are formed, and thus fabrication of a TFT is completed. Such a TFT allows electrons to be transferred through the active layer between the source and drain electrodes.
In addition, after forming a color filter on a lower glass substrate, the upper and lower glass substrates are spaced and attached to each other by sealing. The glass substrates attached thus has a liquid crystal layer therebetween. The liquid crystal layer is formed by injecting a liquid crystal material between the two glass substrates. As a result, an LCD device is completely fabricated.
Operation of the prior art LCD device fabricated thus will be described hereinafter.
With reference to FIG. 1, when a constant voltage is applied to the gate 11-1 of the TFT 11 through the gate bus line 12, a channel for electron transfer is formed in the active layer thereof. Then, if a data signal from the data bus line 13 is supplied to the pixel electrode through the channel, an electron potential is generated between the upper and lower substrate. This is because the liquid crystal layer formed therebetween is used as a dielectric medium. In each pixel, a gradient of liquid crystal is determined in accordance with the amount of data (i.e. voltage level) to be supplied to the pixel electrode and an intensity of a light introduced from a back light also is determined. As a result, a display picture and/or a character can be displayed on a display portion of such an LCD device by distinction of intensity of a light introduced through the transparent pixel electrode. In the TFT of an LCD device, the gate bus line 12 is provided to supply a scanning signal to the gate electrode 11-2 thereof and the data bus line 13 is provided to supply a pixel voltage signal to the source electrode 3.
The above-mentioned LCD device has, as shown in FIG. 1, a matrix structure in which the gate and data bus lines 12, 13 are electrically isolated from each other with an insulating layer (not shown) interposed therebetween and are crossed with each other.
In the prior art LCD device having such a matrix structure, since a data bus line is formed on a gate bus line in addition to a surface of a glass substrate, a step (i.e. height difference) inevitably exists in a crossing position of gate and data bus lines. Thus, disconnection of the data bus line frequently occurs due to a step between the gate and data bus lines.